This invention relates in general to spectrum analyzers and in particular, to a spectrum analyzer and encoder employing spatial modulation of radiation dispersed by wavelength or imaged along a line.
Radiation spectral analysis is presently carried out in a number of ways. Dispersive and Fourier transform based analyzers are for high resolution and can be used for many different applications so that they are more versatile than existing application-specific instruments and procedures. While these analyzers offer superior spectral performance, they tend to be expensive, slow and are not portable. For most applications, these instruments offer a resolution which is largely unnecessary. Many applications require measurements only at several wavelengths so that most of the data taken over the entire complete spectrum using these instruments is discarded and not used at all in the analytical computations. Such analyzers may also be too large and heavy for many practical applications.
In contrast, a non-dispersive approach to spectral analysis employs interference filters of fixed frequency passbands to perform given analytical functions. To perform the measurement, the light signal containing a number of wavelength components is propagated through one or more interference filters which are characterized by a center wavelength and bandwidth. The non-dispersive approach is advantageous over the Fourier transform and dispersive spectrum analyzers in that the non-dispersive approach is less expensive and measures the minimum amount of spectral data required to perform a given analytical function. However, if the analytical function requires a significant number of filters, the system's signal-to-noise ratio is reduced as the total energy measured in a given filter over time is inversely related to the number of filters. Furthermore, if a spectrum analyzer using this approach is configured for a first application, the filters used in the device may have to be replaced, or the number of filters changed, in order to adapt the analyzer to a second application. Therefore, even though the non-dispersive approach may be less expensive and does not measure unnecessary data as compared to the dispersive and Fourier transform approaches, the present non-dispersive approach has clear limitation in adaptability and the number of wavelength which can be analyzed.
Another type of optical spectrum analyzer, which is best described as a hybrid between dispersive and non-dispersive instruments, is the Hadamard spectrometer. The Hadamard spectrometer includes a spatial light modulator, comprised of a disc made of an opaque material with slots therein that reflect or transmit light, where the slots have uniform transmittance or reflectance. A light beam is dispersed according to wavelength onto the disc and the slots are selectively spaced at different radii from the axis to form a number of different optical channels for detecting corresponding wavelength components of the beam. The disc is rotated about the axis and the slots selectively encode the corresponding wavelength components with a binary amplitude modulation. The encoded beam is then directed to a detector. In order to differentiate the intensity of the wavelength component transmitted or reflected by one slot from that of another, the disc is sequentially stepped through a specific number of steps, each step comprised of a binary pattern of open or closed optical channels, which defines one equation in a system of simultaneous equations for the amplitudes of the wavelength components. This set of simultaneous equations is then solved to yield the intensity for each channel prior to any specific analytical function, an approach which is cumbersome and time consuming. Furthermore, as a direct consequence of the binary encoding approach, there is no mechanism by which one can recover the actual signal levels if any one of the signal levels changes substantially over the period of rotation. It should be noted that the system of equation can be simplified if the slots are patterned such that the light is transmitted or blocked one wavelength component at a time. However, this approach changes the optical duty cycle of each of the wavelength components from its optimum value of 50%, thereby degrading the signal to noise ratio. Finally, if a Hadamard analyzer is configured for a first application, and the number of slots is changed to adapt the analyzer to a second application, the data acquisition and decoding algorithms must be changed as well, which significantly limits the instrument's adaptability.
None of the above approaches is entirely satisfactory. It is, therefore, desirable to provide an improved spectrum analyzer where the above-noted disadvantages are avoided or significantly diminished, in particular, where the encoding, data acquisition and demodulation are both generalized and significantly simplified such that the details of the spectrum analyzer can be rendered to a single application specific hardware component.